Premature Birth Clinical Trial
Official title:
Assessing the Impact of Smoke-free Legislation on Perinatal Health in the Netherlands
The purpose of this study is to investigate whether there has been a change in perinatal outcomes following the phased smoking ban introduction (January 2004 for workplaces; July 2008 for bars and restaurants) workplaces in the Netherlands.
Primary research questions
1. Is the phased introduction of smoke-free legislation in The Netherlands associated with
reductions in adverse perinatal outcomes (e.g. perinatal mortality, preterm birth,
small for gestational age)?
2. How do these associations compare with those described for other European countries in
comparable studies (i.e. Belgium (Cox 2013), England (Been et al under review),
Scotland (Mackay 2012))?
Study design Retrospective cohort study (using prospective routinely collected health care
data)
Study population All singleton births in the Netherlands between January 1st 2000 and
December 31st 2011.
Intervention The intervention under study is the ban on smoking in workplaces, and in bars
and restaurants implemented in the Netherlands on January 1st, 2004 and July 1st, 2008,
respectively.
Inclusion and exclusion criteria We will include all registered singleton births in the
Netherlands occurring between January 1st, 2000 and December 31st, 2011. This is the maximum
time period surrounding the ban's introduction for which the required birth data are
available through the data source. Multiple pregnancies, neonates with chromosomal
anomalies, pregnancies with unknown gestational age, pregnancies that ended before 24 weeks
and pregnancies resulting in the birth of a child weighing less than 500 grams will be
excluded.
Outcome
The primary outcomes are:
- Perinatal mortality (stillbirth + early neonatal mortality, i.e. within the first 7
days of life)
- Preterm birth (live birth with gestational age <37+0 weeks)
- Small for gestational age (SGA; live birth with birth weight below 10th centile; The
Dutch PRN reference curves for birth weight by gestational age according to parity, sex
and ethnic background will be used)
To assess whether smoke-free legislation had a selective impact on certain subgroups of
outcomes we furthermore identified a number of secondary outcomes:
- Stillbirth (born dead from 24+0 weeks of gestation)
- Early neonatal mortality (live birth and death within first 7 days)
- Very preterm birth (live birth with gestational age <32+0 weeks)
- Low birth weight (live birth with birth weight <2500 grams)
- Very low birth weight (live birth with birth weight <1500 grams)
- Very small for gestational age (live birth with birth weight below percentile 2.3rd
centile)
- Major congenital anomalies (based on reported associations with antenatal smoke
exposure (Hackshaw 2011)).
Data sources Individual level health care data will be extracted from The Netherlands
Perinatal Registry (PRN). Linked midwifery, obstetric, and neonatal data are available from
2000 to 2011 (including 2011).
Data extraction and handling All relevant variables regarding our outcomes, as well as
relevant potential confounders will be extracted from the database.
Sample size Power calculation for interrupted time series modelling is complicated given the
complexity of the analysis. Similar previous studies have demonstrated statistically
significant and clinically relevant effects of smoke-free legislation on preterm birth (Cox
2013; Mackay 2012; Page 2012), low birth weight (Mackay 2012), SGA (Mackay 2012; Kabir
2013), and perinatal mortality (Mackay et al. and Been et al. both under review). Given the
larger population size of the Netherlands as opposed to the regions in which these studies
were carried out (except for Been et al. under review), we expect our study to have
sufficient power to detect similar effect sizes, should these be present.
Statistical analysis Incidences for each outcome will be presented graphically for each time
period to facilitate visualisation of temporal fluctuations and trends in changes of
incidence levels. To facilitate timing of the events, date of delivery and expected term
date are required for each pregnancy. Data will be presented as outlined in different
Tables. Interrupted time series analyses with adjustment for potential confounders will be
performed to assess the associations between implementation of smoke-free legislation and
primary and secondary outcome measures. Individual-level analysis will be performed using
logistic regression analysis. The models will account for the underlying temporal trend in
incidence, and will allow for a sudden change in incidence ('step change') following the
introduction of the smoking bans. We will test and adjust for any non-linearity in the
underlying time trends, and seasonality will be accounted as appropriate.
Sensitivity analyses (primary outcomes only) In recent years, gestational age is usually
estimated based on early ultrasonography findings, which is more reliable than estimation
based on the last menstrual period. Although the method of ascertainment is not recorded in
PRN, there is an item indicating 'certainty' of the gestational age, which is positive in
about 93% of records. For the primary outcomes preterm birth and SGA we will perform a
sensitivity analysis including only cases in whom gestational age estimation is considered
reliable according to this item.
Recent perinatal management changes have been implemented in The Netherlands resulting in
increased active management of babies born at the edge of viability (i.e. 23-24 weeks
gestation) (NVOG 2010). This has resulted in increased survival at this gestational age as
well as altered management of 25-26 week infants, which likely affects the number of babies
born preterm (although this effect is expected to be small given the small percentage of all
preterm babies being born at this stage), as well as mortality indicators. For the primary
outcomes preterm birth and perinatal mortality we will therefore perform a sensitivity
analysis excluding babies born before 26 completed weeks of gestation.
Smoking during pregnancy is known to be underreported in the PRN database. Definitions
between different caregivers differ; (any) smoking and heavy smoking (>20 cigarettes daily).
We will consider performing subgroup analyses of the impact of smoke-free legislation on the
primary outcomes according to maternal smoking status during pregnancy.
In a sensitivity analysis we will investigate whether smoke-free legislation has any
differential impact on spontaneous preterm birth versus medically indicated preterm birth.
All analyses will be performed using Stata 13.0.
;
Observational Model: Ecologic or Community, Time Perspective: Retrospective
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